This invention relates an ion extraction assembly for generating a beam of ions. In particular, the invention relates to an improved electrode manipulator for such an assembly.
Extraction assemblies are known in the art having several electrodes positioned adjacent to an ion source to extract a beam of ions from the source. Each electrode has a slit through which the beam of ions passes.
WO 97/04474 discloses an ion beam apparatus in which a pair of extraction electrodes are mounted so as to be movable in a first direction towards or away from the ion source (i.e. the direction of the ion beam), and independently movable in a transverse direction (i.e. across the ion beam).
Each of the extraction electrodes is provided with a pair of apertures which can be selectively brought into line with the ion beam upon transverse movement of the electrodes. The apertures have different sizes each defining the extraction optics for a particular range of extraction conditions. While this is an improvement on a conventional electrode assembly, it is still limited to two discrete sets of extraction conditions.
The present invention provides an improvement of this assembly.
According to the present invention, there is provided an ion extraction assembly comprising an ion source, at least one electrode for extracting ions from the source, the electrode having a slit through which a beam of extracted ions passes in use, and an electrode manipulator on which the electrode is mounted, the manipulator having first and second actuators for selectively varying the width of the electrode slit transversely to the ion beam and moving the electrode transversely to the ion beam, and a third actuator to move the electrode in the direction of the ion beam.
By offering control of the width of the slit as well as the position of the electrode, the invention provides a manipulator which allows the electrodes to be used across a wide dynamic range. Across this wide range, the adjustments can be made continuously allowing fine adjustment of the extraction conditions. This is extremely useful in practice where it is becoming increasingly important to be able to switch quickly between a variety of extraction conditions.
The actuators may be arranged such that the first actuator is operative to move the electrode as a whole transversely to the ion beam, and so that the second actuator is operative to move one part of the electrode with respect to the other part which is fixed to control the slit width. Alternatively, the two parts of the electrode may each be movable under the control of a respective first and second actuator in the transverse direction to control both the transverse position and the slit width. However, neither of these alternatives allows independent control of the slit width and transverse position of the electrode, as the movement caused by one actuator to change either the slit width or the transverse position will require a compensatory movement of the other actuator to maintain the desired slit width and transverse position.
Therefore, preferably the first actuator is operative to vary the width of the electrode slit, and the second actuator is operative to move the electrode transversely to the ion beam, the first, second and third actuator arranged to operate independently of one another. This provides independent control of the slit width and transverse position.
As the electrode comprises two parts which are movable relatively to one another, precise alignment of the two parts of the electrode in the direction of the ion beam may be difficult to achieve on assembly. Therefore, preferably the electrode comprises two parts which between them form the slit, wherein part of the electrode is movable relatively to the other part of the electrode in the direction of the ion beam. This allows any misalignment of the parts of the electrode in the beam direction to be corrected. This may be achieved by using the third actuator to move both electrode parts together in the direction of the ion beam and providing a fourth actuator to move one part of the electrode relatively to the other in this direction. Alternatively, the third actuator could move one part of the electrode in the direction of the ion beam and a fourth actuator could move the other part in this direction.
Preferably, the first actuator is mounted on and moveable with one of the second and third actuators, the one of the second and third actuators being mounted on and movable with the other of the second and third actuators. By mounting the first actuator to one of the other actuators, the slit width control can be carried out using a simple actuator while remaining independent of the operation of the actuator on which it is mounted.
The electrode is preferably movably mounted by at least one air bearing. Preferably, the air bearing is a porous plate type air bearing provided with differentially pumped seal. Such bearings have been found to be particularly advantageous in this application as they offer high stiffness and precision, low friction and high speed servo capabilities. This type of air bearing is preferably used at least for the actuator for varying the width of the electrode slit transversely to the ion beam, and for moving the electrode transversely to the ion beam. In order to shield the air bearing from contaminants, when the electrode is supported on at least one shaft, the or each shaft is preferably surrounded by a shield plate movable with the shaft to shield the air bearing in all positions of the electrode.
The first actuator preferably comprises a first motor for driving a first lead screw having a first portion with a screw thread, and a second portion with a screw thread of oppositely handed configuration to the first screw thread, the electrode being provided in two halves, one of which is threadably coupled to the first screw threaded portion, and the other of which is threadably coupled to the second screw threaded portion, so as to be movable in opposite directions upon rotation of the first lead screw.
The first actuator is preferably slidably mounted on a plate, and the second actuator is preferably fixed to the plate and comprises a second motor and a second lead screw threadably coupled to the first actuator in order to move the entire first actuator and electrode transversely to the ion beam upon rotation of the second lead screw.
A pair of air bearing side guides are preferably provided to depend from the plate in order to restrain the electrode to move only in the direction transverse to the ion beam.
The third actuator is preferably fixed to a housing and comprises a third motor driving a third lead screw, threadably coupled to a top plate, the top plate being slidably retained on the housing so as to be movable in the direction of the ion beam, upon rotation of the third lead screw.
The invention also extends to a method of adjusting an electrode in an ion extraction assembly comprising at least one electrode for extracting ions from an ion source, the electrode having a gap through which a beam of extracted ions passes, in use, and an electrode manipulator on which the electrode is mounted, the method comprising the steps of varying the width of the electrode slit transversely to the ion beam, adjusting the electrode transversely to the ion beam, and adjusting the electrode in the direction of the ion beam, all three movements being carried out independently.
The method preferably further comprises, providing at least one porous plate air bearing with a differentially pumped seal to support the electrode, supplying air to the air bearing, and pumping the differentially pumped seal.
The electrode manipulator can be used for any type of electrode where the three degrees of motion provided by the present invention are required. One particular preferred application for the present invention is to a triode assembly comprising a source electrode, and a pair of electrodes being movable together by the electrode manipulator. The invention is equally applicable to a tetrode assembly.